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This is a very common phenomenon on E39 M5's. There are plenty of posts about it. A particular cause has not been pinpointed, though most have resolved the problem by replacing either their PS pump or their steering box. For me, it ended up being a precursor to PS pump failure, so I bit the bullet and bought a new one.
Obviously, the must less drastic diagnosis is PS pump cavitation or use of regular PS fluid as opposed to synthetic ATF. Some seemed to have resolved the phenomenon by increasing the amount of ATF they run or switching to synthetic. Most by replacement of the pump, and only one that I know of by replacement of the box.
--Peter
Obviously, the must less drastic diagnosis is PS pump cavitation or use of regular PS fluid as opposed to synthetic ATF. Some seemed to have resolved the phenomenon by increasing the amount of ATF they run or switching to synthetic. Most by replacement of the pump, and only one that I know of by replacement of the box.
--Peter